SOURCES OF CHANGE 67 



Since the species is the sum of its individuals, it 

 is obvious that these individual responses, adap- 

 tive or otherwise, are as distinctly a part of the 

 species as are those characters which invariably 

 appear in the normal course of development. The 

 two have been contrasted in all of the literature of 

 evolution. Why.^ In order to answer it is necessary 

 to consider the nature of the normal specific char- 

 acters. Ordinarily we say that they are hereditary, 

 but careful thought shows that the organism does 

 not inherit characters, but only the ability to pro- 

 duce them during the course of development.^^ 

 Development takes place normally only in the 

 proper external environment, but the immediate 

 condition to which a given hereditary power re- 

 sponds is in many cases established by the body 

 as development proceeds, so that the relationship 

 with external conditions is indirect. All of us have 

 eyes. The development of eyes cannot precede 

 the differentiation of ectoderm and neural tube. 

 All of us attain stature and mental development 

 within normal limits, but thyroid deficiency or 

 persistence of the thymus exerts a modifying effect. 

 These things are quite in harmony with the princi- 

 ple already expressed, that the major tendency of 

 evolution is toward greater independence of ex- 

 ternal environment. The things which character- 

 ize all individuals of a species are not inherited as 



" Conklin, Sci. Monthly, Vol. VII, p. 499, 1919. 



